Control apparatus



Dec. 18, 1956 CONTROL R. J. EHRET ETAL APPARATUS Filed March 26, 1952 80 F l G. 2 a:

1 1 common SUPPLY W L D, l ST. CONTROL 82 78 2O mm" 5 7 EFFECT 75 42 83 5 5 -Z 2 no. CONTROL 43 7 1; EFFECT 79 I3 9 1 IO 7 Fq 3 RD. CONTROL '2 F EFFECT F I 3 .VANE IN DECREASED REACTANCE OF cou. 2|

sscouo FIRST- usumm. szi'ss V m Rom a zwaaa FIRST ssdono Tumb y ROGER WERNLUND CONTROL CONTROL CONTROL EFFECT EFFECT EFFECT INCREASED INDUCTANCE ATTORNEY.

United States Patet 2,774,879 CONTROL APPARATUS Robert J. Ehret, Philadelphia, and Roger F. Wernlund, Southampton, Pa., assignors to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application March 26, 1952, Serial No. 278,609

12 Claims. (Cl. 250-'-36) The present invention relates broadly to control apparatus of the three-position type wherein the oscillation and non-oscillation of an oscillator portion are caused to provide three distinctly different control effects through the medium of means which effectively shifts the oscillation-controlling value of the impedance of an adjustable element alternately between two different values in synchronism with the alternate energization of two responsive circuit portions of the apparatus. More specifically, the invention relates to an improved form of such control apparatus wherein the means for adjusting the difference between the alternately effective controlling values of the element impedance are so arranged as to prevent the relative adjustment of said two value in an undersirable manner, and wherein one of the responsive portions supplies signals to the oscillator portion which cause the latter to pass from one oscillatory state to the other with a desirable snap action, whereby the responsive portion is caused also to pass from one control effect to another with said snap action.

It is a general object of the present invention to provide improved control apparatus of the type in which three-position control is obtained with an oscillator portion which is adapted to provide three different control effects by the use of means adapted to shift effectively the oscillation-controlling impedance value of an element alternately between two different values in synchronism with the alternate energization of the two responsive portions of the apparatus, the improved apparatus being characterized by the inclusion of novel means for effectively establishing said two controlling impedance values, and novel means for causing the oscillator portion to pass from the oscillating to the non-oscillating state and vice-versa with a desirable snap action as the apparatus is actuated from one control effect to another.

A specific object of the invention is to provide improved apparatus of the type just specified wherein the means which effectively establishes the two alternate oscillation-controlling values is characterized by being arranged so as to prevent the relative adjustment of the two controlling values in an undesirable manner.

Another specific object of the invention is to provide improved apparatus of the general type outlined above wherein the passage of one of two oscillator portions from the oscillating to the non-oscillating state and viceversa is accomplished with a desirable snap action by the use of means adapted to utilize the energization of the respective one of the responsive portions for producing a bias voltage for application to the one oscillator portion.

A more specific object of the invention is to provide. improved control apparatus of the type just specified wherein two alternately operable oscillator portions of the apparatus are supplied with suitable bias voltages commensurate with the state of oscillation of the individual portions, and wherein the voltage developed across the condenser connected in parallel with the operating winding of the responsive device located in one of the reice sponsive portions of the apparatus is utilized to apply an additional bias voltage to the corresponding oscillator portion when the latter is oscillating, and is operative to apply this additional bias voltage at the start of each of the operative periods for the oscillating portion.

Another more specific object of the present invention is to provide improved apparatus of the foregoing type wherein adjustable means is provided for varying the difference between the two effective controlling values of the impedance element of the apparatus, and wherein the adjustable means is adapted to adjust solely the higher one of the two values relative to the lower one thereof, and hence is adapted to adjust said difference in a desirable manner.

In the copending application of Warren M. Gruber, Serial No. 278,565, filed on even date herewith, there is disclosed and claimed three-position control apparatus of the type first specified broadly above and wherein the difference between the two alternately established effective controlling values of the impedance element is adjustable by means which is adapted to adjust the value of solely the lower one of the two controlling values relative to the higher one thereof, thereby permitting the adjustment of said difference to be made in a potentially unsafe and hence undesirable manner. In addition, the apparatus of said Gruber application includes no means for utilizing the charges on the condenser in one of the responsive portions of the apparatus to provide a bias voltage at the start of each period of operation for the corresponding oscillator portion when the latter is oscillating, thereby to cause the apparatus to move from one control effect to another with a desirable snap action.

Accordingly, it is a prime object of the present invention to provide improved three-position control apparatus of the general type disclosed in said Gruber application but which is characterized by the provision of novel and more desirable means for alternately shifting the controlling value of the impedance element, and by the provision of novel bias-producing means for securing snap-action operation when shifting from one control effect to another.

In accordance with the present invention, we provide first and second electronic devices or electron tubes and a common circuit portion interconnecting the input and output circuits of the tubes so as to render them capable of oscillating. The common portion includes an impedance element which is included in the input and output Circuits of the first of the tubes and which is adapted to permit and prevent the oscillation thereof accordingly as the impedance of the element is adjusted to values which are respectively above and below a first critical value individual to that tube. An impedance device is effectively connected in series with the aforementioned element in the input and output circuits of the other, second one of the two tubes and is adapted to oppose the effect of the element with respect to the last mentioned tube for the purpose of requiring a higher adjustment value of the impedance of the element to cause the oscillation of the second tube. In other words, the impedance device is adapted to establish a second critical value for the impedance of the element, which value is individual to the second tube and is higher than that individual to the first one of the tubes, and above and below which the oscillation of the second tube is respectively permitted and prevented. Accordingly, the relays included in the responsive portions of the apparatus which are alternately energized simultaneously with the alternate energization of the two tubes are rendered individually responsive to the state of oscillation of the respective one of the tubes.

In the preferred form of the present invention illustrated herein by way of example, the aforementioned element is an inductive element, and the aforementioned impedance device includes a condenser and an adjustable inductive device connected in series. The adjustable inductive device permits the variation of the opposing effect of the condenser on the element with respect to solely the second tube, and thereby provides a means for adjusting the difference between the alternately established critical values. Further, this adjustment is effected by the adjustment of the higher one of the two critical values relative to the lower one of the two, whereby there is no possibility of the lower one of the two values being adjusted to so low a value as to cause the element to lose control of the state of oscillation of the first tube.

Further in accordance with the present invention, we provide connections between one of the responsive portions of the apparatus and the input or conductivitycontrolling circuit of the respective one of the tubes, whereby the voltage normally developed across the condenser included in said one responsive portion when the corresponding tube is oscillating is utilized to apply a negative grid-cathode bias voltage to the oscillating tube at the start of each period in which the tube is to oscillate, this bias voltage being applied in advance of the normal bias voltage applied to the input circuit of the oscillating tube later during the oscillatory periods thereof. As a result, said tube is caused to go into and out of oscillation, in response to adjustments of the aforementioned element, with a desirable snap action, and the apparatus is therefore caused to pass from one control effect to another with the same desirable snap action.

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described a preferred embodiment of our invention.

Of the drawings:

Fig. l is a circuit diagram of a preferred embodiment of the improved three-position control apparatus of the present invention;

Fig. 2 is a diagram illustrating a preferred manner of interconnecting the responsive devices of the Fig. 1 apparatus; and

Fig. 3 is a diagram which relates the adjustment of the impedance element of the Fig. l apparatus to the three control effects selectively established by the apparatus.

The preferred embodiment of the present invention illustrated by way of example in Pig. 1 is a vane-controlled oscillator controller of the three-position type disclosed and claimed in the aforementioned Gruber application. Broadly, the apparatus includes a first oscillator portion or circuit which controls the operative energization of a first responsive device or relay in accordance with the deviation of the adjusted value of an element from a first critical value, and includes a second oscillator portion or circuit which controls the operative energization of a second responsive device or relay in accordance with the deviation of the adjusted value of the aforementioned element from a second critical value which differs from the first critical value. The element whose value is adjusted to control the state of oscillation of each of the oscillator circuits and the operative energization of each of the responsive devices solely in accordance with the respective one of the critical adjustment values is included in a regenerative coupling circuit portion which is common to the two oscillator circuits. The selective response of the two oscillator circuits and responsive devices to the two, different critical values of the element is accomplished by energizing each of the oscillator circuits from a source of alternating current which is substantially degrees out of phase with the source of alternating current which energizes the other of the oscillator circuits, and by causing the common portion including the aforementioned element to provide one value of regenerative coupling individual to one of the oscillator circuits, and to provide a different value of regenerative coupling individual to the other of the oscillator circuits, for any given adjusted value of the element. The resulting operation of the two responsive devices is such as to establish a first control effect when the adjusted value of the element is above the higher of the two critical values, to establish a second control effect when the adjusted value of the element lies between the two critical values, and to establish a third control effect when the adjusted value of the element is less than the lower of the two critical values, whereby control of the threeposition type is provided.

In accordance with the above, the Fig. 1 apparatus comprises a first oscillator circuit including a triode electron tube 1 and a second oscillator circuit including a triode electron tube 2. Each of the triodes 1 and 2 includes the usual anode, control grid, cathode, and cathode heater elements. The apparatus also includes a common, adjustable regenerative coupling circuit portion 3, a repeater portion 4, and other components to be described hereinafter.

Associated with the first oscillator circuit including the triode 1 is a first electro-responsive relay device 5. The latter is coupled to the oscillator circuit through the repeater portion 4, and is normally effectively responsive to the state of oscillation of solely the triode 1 as will become apparent as the present description proceeds. A second electro-responsive relay device 6 is associated with the second oscillator circuit including the triode 2, and is controlled solely in accordance with the state of oscillation of the triode 2. Broadly, the relay 5 is adapted to be operatively energized, and hence to assume the picked-up position, wherever the triode 1 is oscillating, and is not operatively energized, and hence is maintained in the dropped-out position, whenever the triode 1 is prevented from oscillating. Conversely, the relay 6 is adapted to be operatively energized and to assume the picked-up position whenever the triode 2 is prevented from oscillating, while the oscillation of the triode 2 prevents the relay 6 from being operatively energized and hence causes this relay to assume the dropped-out position.

The relay 5 includes an operating winding 7, movable contacts 8 and 9, and stationary contacts 10 through 13. When the winding 7 is not operatively energized, the contact 8 engages the normally-closed contact 10, and the contact 9 engages the normally-closed contact 12. When the winding 7 is operatively energized, the contact 8 moves into engagement with the normally-open contact 11, and the contact 9 moves into engagement with the normally-open contact 13.

Similarly, the relay 6 includes an operating winding 14, movable contacts 15 and 16, and stationary contacts 17 through 20. When the winding 14 is not operatively energized, the contact 15 engages the normallyclosed contact 1'7, and the contact 16 engages the normally-closed contact 19. When the winding 14 is operatively energized, the contact 15 moves into engagement With the normally-open contact 18, and the contact 16 moves into engagement with the normally-open contact 20.

The common, regenerative coupling circuit portion 3 includes a reactive element or coil 21 which is adapted to have its reactance adjusted to difierent values by a vane 22 which in turn is adapted to be adjusted to various positions within the space formed by the two halves of the coil 21. Broadly, the adjusted reactance value of the {foil 21 relative to a first critical value thereof determines the state of oscillation of the triode 1: that is, determines whether the oscillatirn of the triode 1 is permitted or prevented, while the adjusted reactance value of the coil 21 relative to a second critical value thereof determines the state of oscillation of the triode 2.

To this end, the circuit 3 includes other components which permit the reactance value of the coil 21 to permit and prevent the oscillation of the triodes 1 and 2. Specifically, the circuit 3 includes an oscillator output coil 23, an oscillator input coil 24, a condenser 25, tuning condensers 26 and 27, and other components to be described. The components of the circuit 3 are interconnected with the triode 1 in such a manner as to cause this combination to be a Vane-controlled oscillator of the common-grid type disclosed and claimed in the copending application of Warren Moore, Jr., Serial No. 106,796, which was filed on July 26, 1949, and which issued as Patent No. 2,647,252 on July 28, 1953. Similarly, the components of the circuit 3 are interconnected with the triode 2 in such a manner that this combination also forms a vane-controlled oscillator of the same type. However, the connections of the circuit 3 with respect to the triode 2 include means which are not included in the connections to the triode 1, thereby effec tively establishing the two different critical reactance values for the two triodes.

In accordance with the foregoing, the oscillator output coil 23 is common to the oscillator output or anodecontrol grid circuit of each of the triodes 1 and 2. Specifically, the oscillator output cincuit of the triode 1 can be traced from the anode of the latter through a coupling condenser 28, the coil 23, the condenser 25, the coil 21, a grid bias resistor 29, and conductors 30 and 31 to the control grid of the triode 1. A bypass condenser 32' is connected in parallel with the resistor 29.

The oscillator output :circuit of the triode 2 can be traced from the anode of the latter through the coil 23, the condenser 25, the coil 21, the resistor 29 and parallelconnected condenser 32, the conductors 30 and 31, an adjustable inductor 33, and a condenser 34 to the control grid of the triode 2. A resistor 35 is connected in parallel with the series-connected elements 33 and 34.

The [coil 24 of the circuit 3 is common to the oscillator input or control grid-cathode circuit of each of the triodes 1 and 2. To this end, the oscillator input circuit for the triode 1 can be traced from the control grid thereof through the conductors 31 and 30, the resistor 29 and parallel-connected condenser 32, the coil 21, .the [coil 24, and a conductor 36 to the cathode of the triode 1. Similarly, the oscillator input circuit for the triode 2 can be traced from the control grid of the latter through the condenser 34, the inductor 33, the conductors 31 and 3,0, the resistor 29 and parallel-connected condenser 32, the coil 21, the coil 24, the conductor 36, and a conductor 37 to the cathode of the triode 2.

The condenser 26 is connected between the upper terminal of the coil 23 and the terminal of the resistor 29 which is connected to the conductor 30, while the condenser 27 is connected between the last mentioned terminal and the lower terminal of the coil 24. The coils may advantageously be coupled degeneratively as described in the aforementioned Moore patent.

Each of the triodes 1 and 2 also includes a load or energizing circuit individual to that triode. These circuits are energized with alternating voltages of opposite phase from a power transformer 38 having a primary winding 39, a high voltage secondary winding 40, and a low voltage secondary winding 41. The winding 40 has respective end terminals 42 and 43, while the winding 41 has respective end terminals 44 and 45' and a center-tap connection 46.

Rectifiers 47 and 48 are connected in series between the transformer winding terminals 42 and 43, the negative terminal of the rectifier 47 being connected to the ter minal .42, and the negative terminal of the rectifier 48 being connected to the terminal 43. The positive terminals of the'rectifiers 47 and 48 are connected together at a junction or terminal 49. This terminal is the negative supply voltage terminal for the apparatus, as will become apparent from the following description.

The primary winding 39 of the transformer 38 is connected to supply conductors 50 and 51 which are adapted to supply alternating energizing current to the apparatus. By way of example, the frequency of the alternating supply current is assumed herein to be 60 cycles per second. As shown by the plus signs adjacent the windings 40 and 41, it is assumed herein that the terminal 42 is rendered positive with respect to the terminal 43 during the first half cycles of the supply voltage, during which the terminal 44 is rendered positive with respect to the terminal 45. During the second, alternate halves of the supply voltage cycles, the terminal 43 is positive with respect to the terminal 42, while the terminal 45 is positive with respect to the terminal 44. By virtue of the action of the rectifiers 47 and 48, the voltage of the winding 40 appears as a positive voltage between the terminals 42 and 49 during the first halves of the supply voltage cycles, and appears as a positive voltage between the terminals 43 and 49 during the second half cycles.

The load circuit for the triode 1 can be traced from the anode thereof through a load resistor 52 and a conductor 53 to the transformer winding terminal 43, and from the terminal 49 through a conductor 54, the coil 24, and the conductor 36 to the cathode of the triode 1. Similarly, the load circuit for the triode 2 can be traced from the anode thereof through the coil 23, a conductor 55, and the relay winding 14 to the terminal 42, and from the terminal 49 through the conductor 54, the coil 24, the conductor 36, and the conductor 37 to the cathode of the triode 2. The conductor 54 is connected to ground or to the chassis of the apparatus by a condenser 55'.

In accordance with the foregoing assumptions, the first half cycles of the supply voltage are the only operative half cycles for the triode 2, since it is only during these half cycles that the anode thereof is rendered positive with respect to the associated cathode. Further, the second half cycles are the only operative half cycles for the triode 1, since it is only during these half cycles that the anode thereof is rendered positive with respect to the associated cathode.

It should be readily apparent from the foregoing description that each of the oscillator arrangements formed by the combination of one of the triodes 1 and 2 with the circuit 3 is of the general type disclosed in the aforementioned Moore patent. It is to be understood, however, that the present invention is not limited to the in- IClllSlOll of oscillator circuits of this particular type, but is applicable to other suitable oscillators of known form. However, the type of oscillator disclosed herein is especially well suited for use in the apparatus of the present invention, and has been so disclosed herein for that reason. Since the theory and mode of operation of this type of oscillator are described in detail in said Moore patent, it is deemed to be unnecessary to elaborate on such operation herein except in connection with the overall operation of the invention.

The repeater portion 4 controls the operative energization of the relay 5 in accordance with the presence and absence of oscillation in the apparatus, and hence in accordance with the state of oscillation thereof. As will be described in detail hereinafter, however, the relay 5 is actually controlled in accordance with the state of oscillation of solely the triode 1 under normal operating conditions.

The portion 4 includes triode electron tubes 56 and 57, each of which includes the usual anode, control grid, cathode, and cathode heater elements. The triode 56 has its anode and control grid directly connected together for operation of the triode as a rectifying diode, and will be so designated throughout the remainder of the description. The junction 58 of the resistor 29 and coil 21 in the circuit 3 is connected by a conductor 59 to the anode and control grid of the diode 56, and the cathode of the latter is connected through a condenser 60 and a conductor 61 to the terminal 44 of the transformer winding 46. The latter in turn is connected by a conductor 62 to the negative terminal 49, while the latter is connected by the conductor 54 to the junction 63 at the opposite end of the coil 21 from the junction 58. Accordingly, the diode 56 and the condenser 60 are effectively connected in series across the coil 21 by the circuit just traced, whereby a unidirectional bias voltage of the polarity shown is produced across the condenser 60 whenever either of the triodes 1 and 2 is in oscillation. This voltage results from the rectification of the oscillation signal which is produced across the coil 21 whenever the apparatus is caused to oscillate.

The voltage produced across the condenser 60 is utilized to control the conductivity of the triode 57 which, in turn, controls the operative energization of the relay 5. Specifically, the load or output circuit of the triode 57 can be traced from the transformer winding terminal 43 through a conductor 64 and the relay winding 7 to the anode of the triode 57, and from the cathode thereof through a conductor 65 to the terminal 45 of the winding 46. As previously noted, the terminal 4 thereof is connected by the conductor 62 to the negative terminal 49. Accordingly, it can be seen that the winding 7 is effectively connected in series with the anode-cathode circuit of the triode 57 between the terminals 43 and 49.

The input circuit of the triode 57 can be traced from the control grid thereof through a resistor 66 to the lefthand, positive terminal of the condenser 60, and from the right-hand, negative terminal of the latter through the conductor 61 to the terminal 44 of the winding 41. The terminal 45 of the latter is connected by the conductor 65 to the cathode of the triode 57. From this it can be seen that the input circuit of the triode 57 includes the condenser 60 and the transformer winding 41 connected in series between the control grid and cathode of the triode.

Noting the indicated instantaneous polarities of the transformer secondary winding terminals, it can be seen that the voltage of the winding 41 is a negative grid-cathode bias voltage for the triode 57 which is connected in series with the positive grid-cathode voltage of the condenser 66 in the input circuit of the triode 57. This results from the fact that the terminal 45 of the winding 41 is positive during the operative half cycles for the triode 57 in which the anode of the latter is rendered positive by virtue of the connection to the then-positive terminal 43 of the winding 40.

By virtue of the foregoing connections, the voltage of the winding 41 maintains the conductivity and load current of the triode 57 at a minimum value when there is no oscillation of the apparatus. At this time, the relay is maintained in the dropped-out position since it is not operatively energized. When the apparatus is in oscillation, however, the oscillation signal produced across the coil 21 produces the aforementioned positive control or bias voltage across the condenser 60, whereby the conductivity and load current of the triode 57 are increased sutficiently to effect the operative energization of the relay 5, and hence to cause the latter to assume the picked-up position. The manner in which the relay 5 is controlled solely in accordance with the state of oscillation of the triode 1 will be explained following the description of the operation of the apparatus.

The repeater portion 4 is advantageously included in the apparatus of Fig. 1 for the purpose of providing operation characterized by so-called safe-failure provisions. The use of such an arrangement to provide safe-failure operation, and the safe-failure circuit per se, are not original with us and are not a part of the present invention, but are disclosed and claimed in the copending apetc plication of Robert J. Ehret and Warren Moore, Jr., Serial No. 278,610, filed on even date herewith, and now Patent No. 2,653,279 of September 22, 1953. Since the manner in which the portion 4 provides such safe-failure operation is described in detail in the last mentioned patent, and in the aforementioned Gruber application, only brief reference to such operation will be presented herein in the subsequent description of the operation of the present invention.

The cathode heaters of the triodes 1 and 2 are energized by being connected in series between the terminals 44 and 45 of the winding 41 by partially-shown conductors. Similarly, the cathode heaters of the tubes 56 and 57 are also connected in series between the terminals 44 and 45 by partially-shown conductors.

Insofar as it has been described, the apparatus of Fig. 1 does not differ significantly from the apparatus disclosed in the aforementioned Gruber application except in regard to the connections between the circuit 3 and the input circuits of the triodes 1 and 2. As mentioned earlier, the present invention distinguishes from the Gruber apparatus in regard to these connections, by virtue of which the apparatus of the present invention is not subject to a disadvantageous characteristic of the apparatus of the Gruber application. The nature of this distinction will be elaborated on more fully hereinafter following a description of the operation of the present invention.

Since the winding 14 of the relay 6 is included directly in the load circuit of the triode 2, the operative energization of the relay 6 is controlled directly by the state of oscillation of the triode 2. Thus, the relay 6 remains in the dropped-out position when the triode 2 oscillates during its operative half cycles, and is operatively energized and actuated into the picked-up position when the oscillation of the triode 2 is completely prevented.

For the purpose of illustrating the operation of the control apparatus of the present invention to best advantage, we have chosen to illustrate the apparatus of Fig. 1 as being actuated by a galvanometer or millivoltmeter type of instrument 66 which is responsive to the temperature of a thermocouple 67. To this end, the thermocouple 67 is connected by conductors 68 and 69 to the instrument 66, and the pointer 70 of the latter is connected by a suitable mechanical linkage 71 to the vane 22. A so-called thermocouple burnout circuit including a resistor 72 and a reactifier 73 connects the voltage of the lower half of the transformer winding 41, between the terminal 45 and the connection 46, across the instrument 66 in series with the elements 72 and 73. This circuit is operative to cause the pointer 70 to move up-scale in the event that the thermocouple circuit should become open, and thereby causes such a failure to produce the same response of the apparatus as would be effected by a lar e increase in the temperature of the thermocouple.

The diagrams of Figs. 2 and 3 Figs. 2 and 3 are diagrams relating to the several control effects which the apparatus of Fig. 1 is operative to produce in the presence of various adjusted reactance values of the coil 21 corresponding to various positions of the vane 22 and to various temperatures of the thermocouple 67. Fig. 2 illustrates an advantageous manner of interconnecting the contacts of the relays 5 and 6 so that each of the three control effects produced by the apparatus will connect a respectively different one of three terminals 74, 75, and '76 to a supply terminal 77, and will cause the appropriate actuation of signal lamps 78 and 79. Fig. 3 illustrates the relationship between the adjusted reactance value of the coil 21, the two critical reactance values, and the three control effects selectively established by the apparatus.

As shown in Fig. 2, a supply terminal 80 is directly connected to a common control terminal 81, while the other supply terminal 77 is connected to the movable contact 8 of the relay 5. The normally-closed contact 10 of the latter-is connected to the third control effect terminal 76. The normally-open contact 11 of the relay 5 is connected to-the movable contact 15 of the relay 6, while the normally-open contact 18 of the latter is connected to the second control effect terminal 75. The first control effect terminal 74 is connected to the normally-closed contact 17 of the relay 6.

As a result .of the foregoing connections, the terminal 74 alone is energized from the supply terminal 77 whenever the first control etfect is established in the apparatus, such energization being produced by the relay 5 being in the picked-up position and the relay 6 simultaneously being in thedropped-out position. The terminal 75 alone is energized whenever the second control effect is established, at "which time both of the relays 5 and 6 are in the picked-up position. Finally, the terminal 76 alone is energized whenever the third control effect is established, at which time the relay 5 is in the dropped-out position while the relay 6 is in the picked-up position. Each of the relays 5 and 6 is shown in the dropped-out position in Figs.1and 2.

The upper terminal of the signal lamp 78 is connected through a resistor 82 to the transformer winding terminal .42, while the terminal 43 of the transformer is connected to one terminal of the lamp 79. The remaining terminals of the lamps 78 and 79 are connected at a junction 83 which in turn is connected to the movable contact 9 of the relay 5. The normally-closed contact 12 of the latter is connected to the terminal of the lamp 79 which is connected to the terminal 43.

The terminal of the lamp '78 which is connected to the resistor 82 is connected to the movable contact 16 of the relay 6, while the normally-closed contact 19 of the latter is directly connected to the normally-open contact 13 of the relay 5. The mannerinwhich these connections serve to provide the actuation of the proper signal lamp for the various control effects will be apparent from the description of the operation of the apparatus to be made hereinafter.

The operation of the apparatus The specific operationof the novel means for .obtaining the alternately difierent critical reactance values for the coil 21 will first be described. In this connection, it should be noted that the circuit 3 acts as a common, adjustable regenerative coupling means for the triodes 1 and 2. The magnitude of the regenerative coupling provided by the circuit 3 is determined by the adjusted reactance value of the coil 21, and there is a critical value of this coupling above which the triodes will oscillate and below which they willbe prevented from oscillating. However, because of the presence of the condenser 34 and inductor 33, a given adjusted reactance value of the coil 21 will result in a lower value of coupling with respect to thetriode 2. In other words, there is a higher critical reactance value of the coil 21 for the triode 2 than there is for the triode 1. The reason for this is that the capacitive reactance of the condenser 34 reduces the net inductive reactance of the coil 21 with respect to the triode 2, and hence causes a given adjusted reactance value of the coil 21 to produce a lower net value of inductive reactance for the triode 2 than is produced for the triode 1.

Stating theabove in a different manner, itis the net inductive reactance betweenthe point 63 inthe circuit 3 and the control grid of the triode 1 which determines whether that triode is or is not oscillating during its operative half cycles. As a result, a given adjusted-value of the reactance of the coil 21, which coil is seen to be connected between the point 63 and each of said control grids, produces a higher net inductive reactance between the point 63 and the control grid of the triode 1 than it produces between the point 63 and the control grid of the triode 2, due to the presence of the capacitive 16 reactance of the condenser 34 in the connection to the triode 2,

The inductive reactance of the inductor 33 opposes the capacitive reactance of the condenser 34 and aids the inductive reactance of the coil 21, all with respect to solely the triode 2. Accordingly, the amount by which the condenser 34 reduces the net inductive reactance individual to the triode 2 is determined by the adjustment of the inductor 33. Thus, if the inductor 33 is so adjusted that the inductive reactance thereof is just equal to the capacitive reactance of the condenser 34, the net reactance for the triode 2 will be equal to that for the triode 1, which is the adjusted reactance value of the coil 21. Under this condition, there will be no difference between the critical reactance values for the two triodes, and the neutral zone or zone between these two critical values will have been reduced to zero.

As the inductance of the inductor 33 is decreased below this value, which is advantageously made the maximum adjustment value of the inductor, higher values of the adjusted reactance value of the coil 21 will be required to cause the oscillation of the triode 2, due to the opposing action of the condenser 34. Thus, the decrease of the inductance of the inductor 33 will widen the neutral zone by increasing the value of the higher of the two critical reactance values.

Conversely, as shown in Fig. 3, the neutral zone will be made narrower by increasing the inductive reactance of the inductor 33. The advantages obtained with this arrangement which are not provided by the apparatus of the aforementioned Gruber application will best be seen following the illustration of the operation of the present invention now to be made.

With reference to Figs. 1 through 3, let it be assumed that the terminals 74, 75, 76, and 81 of Fig. 2 are connected to means adapted to supply heat to the space in which the thermocouple 67 is located. Thus, when this temperature, which will be called the measured temperature, is below a desired, Set-point value, the vane 22 will be out from between the halves of the coil 21, but will move toward the coil 21 and into said space as the measured temperature is increased. Let it also be assumed that the heating means supplies heat at a maximum rate when the first control efiect terminal 74- is energized from the supply terminal 77, that said means supplies heat at an intermediate rate when the second control effect terminal 75 is energized, and that said means supplies heat at -a minimum, safe rate when the third control effect terminal 76 is energized.

In accordance with the above assumptions, when the measured temperature is low, and is weil below the desired value, the vane 22 will be well out from between the halves of the coil 21, the adjusted reactance value of the latter will be high, and both of the triodes 1 and 2 will be in oscillation, each during its own operative half cycle of the supply voltage. Accordingly, the relay 5 will :be operatively energized and will be in the picked-up position, since the triode 57 will be biased to have high conductivity. The relay 6 will not be operatively energized, however, and will be in the dropped-out position, since the conductivity of the triode 2 will be low because of the oscillation thereof. Therefore, the first control effect will be established in the apparatus, with the relay 5 picked-up and the relay 6 dropped-out. As can be seen from Fig. 2, the terminal 74 is energized under these conditions, and the lamp 79 alone is illuminated. Further, the rate of heat supply will be maximum, as it should be for the condition of a low value of the measured temperature.

As the measured temperature increases in the presence of the supplied heat, the vane 22 will move toward. the coil 21. This will decrease the adjusted reactance of the latter and will subsequently cause the triode 2 to stop oscillating as the reactance value of the coil 21 is reduced below the second, higher critical reactance value individual to the triode 2. This will cause the relay 6 to be operatively energized and to be actuated into the picked-up position, while the relay will remain pickedup in the presence of the continued oscillation of the triode 1. ACcOrdingly, the second control effect will be established at this time, whereby the terminal 75 will alone become energized. Both of the lamps 78 and 79 will then be illuminated to indicate the establishment of the second control effect of the apparatus, and the supply of heat will be reduced to the intermediate value.

A further increase in the measured temperature will cause a further reduction in the adjusted reactance value of the coil 21, and the oscillation of the triode 1 will subsequently be prevented as the reactance of the coil 21 is reduced below the lower critical reactance value individual to the triode 1. This will cause the relay 5 to move to the dropped-out position, since there will no longer be any oscillation to cause a positive bias voltage to render the triode 57 sufficiently conductive to operatively energize the relay 5. Since the relay 6 will remain in the picked-up position, however, the third control effeet will be established, and solely the terminal 76 will become energized. In addition, only the lamp 78 will be illuminated, and the rate of heat will be reduced to the minimum or safe value.

A subsequent decrease in the measured temperature will cause the apparatus to pass from the third control effect back into the second control efiect, since the reactance of the coil 21 Will be increased above the lower critical value, the triode 1 will be caused to oscillate, the triode 57 will be rendered more conductive, and the relay 5 will be actuated into the picked-up position. This will cause the rate of heat supply to be increased to the intermediate value. If the measured temperature continues to decrease, the apparatus will subsequently pass from the second control effect to the first control effect, since the reactance of the coil 21 will be increased above the higher critical value individual to the triode 2, the latter will be caused to oscillate, the relay 6 will move to the dropped-out position, and the relay 5 will remain pickedup. This will cause the heat supply to be increased to the maximum rate as needed at that time.

It is noted that, in practice, the width of the neutral zone is usually made desirably small and is usually made to embrace the desired value of the measured and controlled temperature, whereby the apparatus is operative to regulate the rate of heat supply as necessary to maintain the temperature at or nearly at the desired, set-point value.

The manner in which the triode 57 and relay 5 are responsive to the state of oscillation of solely the triode 1 should be apparent from the foregoing. Specifically, in normal operation, the triode 2 can never be oscillating except when the triode 1 is also oscillating, each-during its own operative half cycles, and therefore the triode 2 has always stopped oscillating before the triode 1 is prevented from oscillating. Since, therefore, the triode 1 is always the last to be prevented from oscillating and is the first to be permitted to oscillate upon movement of the vane 22, the state of oscillation of solely the triode 1 controls the operative energization of the relay 5.

It is noted that the relay winding 7 has a condenser 84 connected thereacross, while a condenser 85 is similarly connected across the relay winding 14. In each case, the presence of the respective condenser prevents a relay which is operatively energized and actuated into the picked-up position during its operative half cycle from moving to the dropped-out position during the intervening, inoperative half cycles.

It is also noted that, in order to secure the operation described above, the time constant of the grid bias combination of the elements 29 and 32 must be short compared to a half cycle of the alternating energizing voltage in order that each triode may oscillate or not oscillate during its operative half cycles without any interference from the bias voltage developed in accordance with the state of oscillation of the other triode during its intervening operative half cycles. Thus, the negative gridcathode bias voltage developed across the resistor 29 during the second half of a supply voltage cycle when the triode 1 is oscillating must not be present during the succeeding first half cycle, since a negative bias voltage at that time would cause the improper deenergization of the relay 6 if it happened that the triode 2 was prevented from oscillating at that time.

The operation described above is seen to be of the safe-failure type, as previously mentioned, since it does not cause the establishment of the first or high heat control effect upon the failure of the relays 5 and 6 to be energized at the same time. It can be seen clearly from Fig. 2 that the simultaneous assumption of the droppedout position by the two relays merely energizes the minimum heat terminal 76, and thus does not create an unsafe control condition. A further, more detailed description of the safe-failure aspects of the Fig. 1 apparatus will be found in the aforementioned Gruber application and Ehret and Moore patent as previously noted.

The resistor 35 is connected across the elements 33 and 34 in order to prevent the occurrence of parasitic oscillations in the apparatus, and to provide a D. C. path to the grid of the triode 2. A condenser 86, connected across the triode 2 between the anode and cathode thereof, parallels the anode-cathode capacitance of the triode 2 and hence improves the stability of the operation of the apparatus and minimizes instability due to tube changes. The condenser 86 is a degenerative element with respect to each of the oscillator triodes 1 and 2, and may be either an actual condenser or a condenser composed of various stray and distributed capacitances present in the circuit. When it is an actual condenser, the condenser 86 may advantageously be made adjustable, as shown in Fig. 1, to permit the aforementioned critical values of reactance for the coil 21 to be shifted together over a predetermined range.

It is noted that the aforementioned condenser 27 may also be either an actual condenser or a condenser composed of various stray and distributed capacitances present in the circuit. When the condenser 27 is an actual condenser, it may advantageously be made adjustable to permit the tuning of the coil 24 relative to the coil 23 for dead spot adjustment purposes as explained in said Moore patent.

The above operating example should make readily apparent the advantageous operation obtainable with the present apparatus by virtue of the inclusion of the elements 33 and 34. That is, the adjustment of the in ductor 33 to lower its inductance and increase the width of the neutral zone merely raises the higher critical value of the reactance of the coil 21. Therefore, if the neutral zone width is increased excessively, the result is merely that the reactance of the coil 21 can never be increased sufiiciently by the vane 22 to cause the oscillation of the triode 2 and the establishment of the first control effect. Accordingly, such an excessive increase in the neutral zone widthmerely prevents the apparatus from ever causing heat to be supplied at the maximum rate, and hence does not cause the establishment of an unsafe and therefore undesirable control effect. This constitutes an important improvement over apparatus, such as that disclosed in said Gruber application, wherein the lower critical reactance value can be decreased to the extent that the apparatus can never pass from the second control efiect to the third, no matter how high the measured temperature increases and how much the reactance of the coil is reduced.

The means for providing snap action of the apparatus as the latter is actuated from the first control effect to the second control etfect and vice-versa will now be described. In this connection, it is noted that the triode 2 must be reminded at the beginning of each of its operative half cycles as to whether it was in oscillation or not during the previous operative ihalfj'cycle"if snap action operation is to be had. T e an reminded? is properly employed in this eel ne' since each operative half cycle for the triode' Zfisfseparated from the other operative half cycles therefor by the 'interveiiing operative half cycles for the trio'de 1.

The reason that such:re'n'1inding is opera'ti've'to provide such snap action operation "can 'betracedto the fact that less reactance of the coil 21is heeded to "cause o'scillatien of the triode 2 as the ride'f thetri'ode 2 is biased more negatively 'with respectto "its cathode.

However, as was noted above, itis'h'ot possible to permit the grid b'ia's components 29and=32te=provid such a memory effect, since this would 'terfer e with the desired independent alternate operation of the two tri'odes. We have dis'covered, however, that the memory effect required to obtain snap actionoperation can beiprovided by utilizing the chargeor volt'a-g'e'maintained fromoiie cycle to the next on therelay condenser 85 to remind the triode 2 as to its state of oscillation in the preceding operative half cycle. To this end, the control grid conductor 30 is connected by way of a resistor 87 to the transformer side of the relay condenser 85, and is connected by way of a resistor 88 to the transformer winding terminal 43.

By virtue of the connections just specified, and others of the circuit paths within'the apparatus, it appears that the voltage storedon the condenser 85 causes a negative bias voltage to be applied-to the control grid-cathode circuit of the triode 2 at the 'start of each operative half cycle therefor when the oscillation of the triode 2 is permitted by the coil '21. In other words, the relatively low voltage drop produced across the condenser-85 when the relay6 is notoperatively energized appears to be effective to cause a negative grid-cathode bias voltage to be applied to the then oscillating triode 2 which is re sponsible for the deenergized condition of the relay, this application taking place effectively at'the start of each operative half cycle for the triode 2. Thus, the memory function necessary to snap action operation of the delay 6 is provided, and chatter-free operation thereof is obtained. I

Although the regular grid bias voltage is developed by the components'29 and 32 shortly after thestart of each operative half cycle forthe triode 2'when the latter is permitted to oscillate, it hasbeen found that this bias voltage is developed too late in each half cycle to provide snap action operation, and that the earlier bias voltage application effect provided by the relay condenser voltage through theconnections described above is necessary if snap action operation of the relay 6 is to be had.

It is noted that the repeater portion 4 endows the operation. of the relay 5 with a desirable snap 'action characteristic, thereby making it unnecessaryto' apply the above principle to the circuits which control the operative energiation of the relay 5.

We believe it to be apparent that the foregoing description of the illustrated preferred embodiment of the present invention clearly demonstrates the manner in which the invention fulfills the several objects stated earlier herein.

By way of illustration and example, and not by way of limitation, it is noted that a Working model of the apparatus of Figs. 1 through 3 employed the following values for the components of the apparatus:

Resistor 29 220K ohms. Resistor 35 3900 ohms. Resistor 52 18K ohms. Resistor 66 6.8 megohms. Resistor 82 180K ohms. Resistor 87 18 megohms. Resistor88 l8 megohms. Condenser25 5000 mmf. Condenser 26 25 mmf.

Condenser 27 l0 mmf. (approx). Condenser 28 1000 mmf. Condenser 32 4700 mmf. Condenser 34 25 mmf. Condenser 55 1 mf.

Condenser 60 5000 mmf. Condenser 84 10 mf.

Condenser 85 l0 mf.

Condenser 86 2-6 mmf. (approx.). Inductor 33 l-4 microhenries (approx.) Coil 21 0.51 microhenries. Coil 23 2 microhenries. Coil 24 2 microhenries. Coupling 23-24 0.5 microhenries. Relay winding 7 5000 ohms.

Relay winding 14 5000 ohms.

Trans. winding 40 264 volts.

Trans. winding 41 12.6 volts.

Tube 1 /2 l2AU7.

Tube 2 /2 l2AU7.

Tube 56 /& l2AU7.

Tube 57 /2 l2AU7. Supply voltage frequency 60 C. P. S.

While, in accordance with the provisions of the statutes, we have illustrated and described the best form of the invention now known to us, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is as follows:

1. Control apparatus, comprising first and second electronic devices each having input and output circuits, at single adjustable impedance element common to said circuits and connected in an oscillatory feedback connection within the input and output circuits of said first device and operative to permit and prevent the oscillation thereof accordingly as the impedance of said element isadjusted to values respectively above and below a first critical value individual to said first device, an impedance device effectively connected in series with said element in an oscillatory feedback connection within the input and .output circuits of solely said second device and operative to oppose the effect of said element with respect to said second device and hence to establish a second, higher critical value for the impedance of said element which is individual to said second device and above and below which the oscillation of the latter is respectively permitted and prevented, first and second responsive circuit portions of said apparatus, means adapted to connect energizing means to said first and second portions and to said first and second devices and operative to permit the oscillation of said first device but to prevent the oscillation of said second device during one half of each cycle of an alternating energizing voltage, and to prevent the oscillation of said first device but to permit the oscillation of said second device during the other half of each of said cycles, first and second electro-responsive means respectively individual to said first and second portions, and means operative to control the operative energization of each of said responsive means in accordance with the state of oscillation of the respective device and hence in accordance with the deviation of the impedance value of said element from the respective one of said critical values.

2. Apparatus as specified in claim 1, wherein the state of oscillation of said first device during said first half cycles is determined by the magnitude of the net inductive reactance between a first point on said element which is common to the input and output circuits of both of said first and second devices and a second point on said element which is individual to the input and output circuits of said first device, wherein the state of oscillation of said second device during said second half cycles is determined by the magnitude of the net inductive reactance between said first point and a third point which is individual to the input and output circuits of said second device, wherein said element is connected between said first and second points and said element and said impedanc device are connected in series between said first and third points, and wherein each of said first and second devices requires substantially the same magnitude of net inductive reactance between the respective ones of said points to cause oscillation of the device.

3. Apparatus as specified in claim 2, wherein said element is an inductive element having inductive reactance the magnitude of which is determined by the adjustment of said element, wherein said impedance device includes a condenser having capacitive reactance, and wherein the latter renders the magnitude of the net inductive reactance between said first and third points substantially smaller than the magnitude of the net inductive reactance between said first and second points for any adjusted value of the reactance of said element.

4. Apparatus as specified in claim 3, wherein said impedance device also includes an adjustable inductive device having inductive reactance the magnitude of which is determined by the adjustment of said inductive device, wherein the latter is connected in series with said condenser and said element between said first and third points, and wherein the difference between the magnitudes of the net inductive reactance between said first and second points and said first and third points is determined by the adjusted value of said inductive device.

5. Apparatus as specified in claim 1, wherein each of said first and second devices is a triode electron tube having anode and control grid elements included in the respective one of said output circuits and having a cathode element which is included with said control grid in the respective one of said input circuits, wherein said element is connected between the anode and cathode of said first tube and the control grid thereof and is operative to determine the state of oscillation of said first tube during said first half cycles solely in accordance with the adjusted impedance value of said element, and wherein said element and said impedance device are connected in series between the anode and cathode of said second tube and the control grid thereof and are operative to determine the state of oscillation of said second tube during said second half cycles in accordance with the resultant of the adjusted impedance value of said element and the impedance value of said device.

6. Apparatus as specified in claim 5, wherein said element is an inductive element having inductive reactance the magnitude of which is determined by the adjustment of said element, wherein said impedance device includes a condenser having capacitive reactance, wherein the latter renders the magnitude of the net inductive reactance between the anode and cathode and the control grid of said second tube substantially smaller than the magnitude of the net inductive reactance between the anode and cathode and the control grid of said first tube, and wherein each of said tubes requires substantially the same magnitude of said net inductive reactance to cause oscillation of the tube.

7. Apparatus as specified in claim 6, wherein said impedance device also includes an adjustable inductive device having inductive reactance the magnitude of which is determined by the adjustment of said inductive device, wherein the latter is connected in series with said condenser and said element between the anode and cathode and the control grid of said second tube, and wherein the difierence between the magnitudes of the net inductive reactance individual to said first and second tubes is determined by the adjusted value of said inductive device.

8. Control apparatus, comprising first and second electronic devices, each having input, output, and load circuits, a single adjustable impedance element common to said input and output circuits and connected in an oscillatory feedback connection within the input and output circuits of both of said devices and operative to permit and prevent the oscillation of said first device accordingly as the impedance of said element is adjusted to values which are respectively on one or the other side of a first critical value, and to permit and prevent the oscillation of said second device accordingly as the impedance of said element is adjusted to values which are respectively on one or the other side of a second critical value, energizing means adapted to connect the load circuits of said devices to a source of alternating current of a predetermined frequency and operative to permit the oscillation of said first device and to prevent the oscillation of said second device during the first half of each cycle of the alternating current, and to prevent the oscillation of said first device and to permit the oscillation of said second device during the second half of each of said cycles, bias means included in said input circuits and operative to develop a negative bias voltage during those of said half cycles in which one of said devices is oscillating, and operative to apply said bias voltage to the input circuit of the oscillating device during the oscillatory half cycles therefor, the current flow in each of said load circuits thereby being in joint accordance with the energization thereof and the state of oscillation of the respective device, a first electro-responsive means connected in the load circuit of said first device, a second electroresponsive means included in a circuit portion in which the current flow is in accordance with the state of oscillation of said second device, each of said first and second devices being operative to control the operative energization of the respective one of said responsive means in accordance with the state of oscillation of the respective device and hence in accordance with the deviation of the impedance of said element from the respective one of said critical values, a charge storing means connected in the load circuit of said first device and operative to have produced thereacross a voltage of a value determined by the state of oscillation of said first device, and means operative to couple said charge storing means to the input circuit of said first device and to cause said charge storing means to apply a negative bias voltage to the last mentioned input circuit at the start of each oscillatory half cycle for said first device.

9. Apparatus as specified in claim 8, wherein said first responsive means includes an operating winding connected in series in the load circuit of said first device, and wherein said charge storing means is a condenser which is connected in parallel with said winding.

10. Apparatus as specified in claim 9, wherein each of said devices is a triode electron tube having anode and cathode elements included in the respective one of said load circuits and having a control grid element which is included with said cathode in the respective one of said input circuits, and wherein said bias means includes a single resistor connected between the cathode and control grid of each of said tubes and includes a condenser connected in parallel with said resistor.

11. Apparatus as specified in claim 10, wherein said energizing means includes a transformer winding adapted to have an alternating voltage of said frequency induced between its terminals, first and second diode rectifiers connected in series between said terminals with the positive elements of the rectifiers joined at a first junction, and first and second resistors connected in series between said terminals and joined at a second junction, wherein the load circuit of said first tube includes a conductor connecting said parallel connected winding and condenser between the anode of said first tube and one of said terminals, and wherein there are included means connecting said first junction to the cathode of said first 17 tube, and means connecting said second junction to the control grid of said first tube.

12. Control apparatus, comprising first and second electronic devices, each having input, output, and ioad circuits, a single adjustable impedance element common to said input and output circuits and connected in an oscillatory feedback connection 'within the input and output circuits of said first device and operative to permit and prevent the oscillation thereof accordingly as the impedance of said element is adjusted to values respectively above and below a first critical value individual to said first device, an impedance device effectively connected in series with said eleemnt in an oscillatory feedback connection within the input and output circuits of solely said second device and operative to oppose the effect of said element with respect to said second device and hence to establish a second, higher critical value for the impedance of said element wh ch value is individual to said second device and above and below which the oscillation of the latter is respectively permitted and prevented, energizing means adapted to connect the load circuits of said first and second devices to a source of alternating current of a predetermined frequency and operative to permit the oscillation of said first device and to prevent the oscillation of said second device during the first half of each cycle of the alternating current, and to prevent the oscillation of said first device and to permit the oscillation of said second device during the second half of each of said cycles, bias means included in said input circuits and operative to develop a negative bias voltage during those of said half cycles in which one of said first and second devices is oscillating, and operative to apply said bias voltage to the input circuit of the oscillating device during the oscillatory half cycles therefor, the current flow in each of said load circuits thereby being in joint accordance with the energization thereof and the state of oscillation of the respective device, a first electroresponsive means connected in the load circuit of said first device, a second electro-responsive means included in a circuit portion in which the current flow is in accordance with the state of oscillation of said second device, each of said firstand second devices being operative to control the operative energization of the respective one of said responsive means in accordance with the state of oscillation of the respective device and hence in accordance with the deviation of the impedance of said element from the respective one of said critical values, a charge storing means connected in the load circuit of said first device and operative to have produced thereacross a voltage of a value deter-ruined by the state of oscillation of said first device, and means operative to couple said charge storing means to the input circuit of said first device and to cause said charge storing means to apply a negative bias voltage to the last mentioned input circuit at the start of each oscillatory half cycle for said first device.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,791 Wild Dec. 28, 1948 2,458,283 McCreary Jan. 4, 1949 2,468,138 Terry Apr. 26, 1949 2,531,313 Wannamaker Nov. 21, 1950 2,584,728 Michel Feb. 5, 1952 

